1. Field of the Invention
This invention relates to air-operated diaphragm brakes of the type including a push-rod actuator and used with air brake systems on vehicles such as trucks. More particularly, the invention relates to a dust shield for preventing contaminants from entering the service brake chamber which also serves as a guide to maintain the push-rod in proper adjustment with respect to the slack adjuster.
2. State of the Prior Art
A typical air brake system for a vehicle such as a bus, truck or the like includes separate brake means such as a brake shoe and drum at the vehicle wheels. The brake shoes are actuated in one direction to apply the brakes and in the other direction to release the brakes. The brake means may serve as both service and emergency brakes, and individual brake actuators are located adjacent the wheels for actuation of the brake means.
A typical brake actuator includes a service brake actuator for applying and releasing the brakes in response to delivery and exhaust of compressed air. Often a spring brake actuator is disposed in tandem with the service brake actuator. The spring brake actuator uses the force of a strong compression spring to operate the service brake actuator and to apply the brakes when air in a spring chamber is reduced below some predetermined level. Air may be reduced in the spring brake actuator under the control of the operator in the manner of a parking brake, or automatically in emergencies such as a failure of portions of the brake air system.
Brake actuators typically employ diaphragms that move in response to air pressure and springs. The diaphragms in turn move push-rods in reciprocating fashion which move the brake means between operative and inoperative positions. Typically, the push-rod is pivotally attached to a slack adjuster which is splined to a camshaft on which a cam causes the brake shoes to be forced against the brake drum as the cam is rotated. The slack adjuster acts as a lever to amplify the force generated by air pressure in the brake actuator.
The push-rod typically extends through an aperture in the brake actuator. Because the aperture is in open communication with the interior of the brake actuator, dust shields are frequently employed to prevent contaminants from entering the interior of the brake actuator. Such dust shields are typically flexible neoprene or rubber boots which move as the push-rod reciprocates and articulates.
Under heavy use, the push-rod sometimes abrades against the edges of the aperture which causes undue wear on the push-rod and may lead to premature failure. Also, when the push-rod is in the inoperative position, i.e., when the brakes are released, the push-rod may not be in the proper position for an automatic slack adjuster to maintain the brakes at the optimum adjustment. Typically, a preselected optimum angle is determined between the slack adjuster and the push-rod when the push-rod is in the inoperative position, and if this angle is not maintained, the automatic slack adjuster may maintain inaccurate adjustments.